Preheat the oven to 120ºC. Whisk the eggs and sugar together in a heat-prof bowl over a saucepan of simmering water until the sugar dissolves. Add the chopped chocolate, sour cream and frangelico or brandy, and mix until chocolate has melted. Pour mixture into a 20cm x 20cm square cake tin lined with baking paper. Place in oven and cook for 45 minutes. It should be a little wobbly in the centre when ready as it will continue to cook in its own heat. Remove from oven and leave to cool completely. Once cooled to room temperature, place in the fridge to make it easier to cut. Using a sharp knife (run under hot water and then dry), cut the fudge into 3cm squares. Dust with cocoa and place on a serving plate.